1. Field of the Invention
The present invention relates to a novel process for copolymerizing butadiene and styrene and to a process for preparing improved butadiene-styrene copolymers having a high cis-1,4 configuration of the butadiene units.
2. Description of the Related Art
High cis-1,4 polybutadiene rubber has many excellent properties such as high resilience, low heat-built-up and abrasion resistance, and is therefore used for tires.
However, this rubber tends to have undesirable processing characteristics such as poor banding on a roll mill, which therefore necessitates a multiplation mixing stage. Further, the rubber has poor tear resistance, which results in chipping or cutting of the rubber.
A butadiene-styrene copolymer, which has a high cis-1,4 configuration of the butadiene units and the suitable contents of styrene, may be the most desirable polymer to solve these drawbacks without adversely affecting the above-described excellent characteristic properties of cis-1,4 polybutadiene.
Further, it has been known in the prior art that polybutadienes having a high cis-1,4 configuration of the butadiene units can be produced with high activity catalysts such as a three-component catalyst system comprising a nickel compound, a Lewis acid and an organo aluminum compound, as described in U.S. Pat. No. 3,170,904 and others.
The copolymerization of butadiene and styrene was carried out with an aforementioned three-component catalyst having acetylacetone nickel compound as the nickel compound, titanium tetrachloride as the Lewis acid and triethylaluminum as the organo aluminum compound, as described above (U.S. Pat. No. 3,639,520 and Kogyo Kagaku Zasshi, Vol.72, p. 2081 (1969)).
In this case, the polymerization activity of the catalyst system becomes markedly lower. Further, resultant copolymers have a high cis-1,4 configuration of the butadiene units and have a very low content of bound styrene of the copolymer. As the resultant copolymers contain homopolystyrene, the desirable properties of the copolymer deteriorate.
Further, there are processes, which are well-known in the industry, for producing butadiene-styrene copolymers.
Main catalyst systems in such processes are as follows: an alkyllithium compound system and a radical type catalyst system. Further, in the well-known processes mentioned above, the cis-1,4 configuration of the butadiene units of the copolymer is less than or equal to about 60%.